Spider silks are protein polymers that display extraordinary physical properties. Among the different types of spider silks, draglines are most intensely studied. Dragline-silks are utilized by orb weaving spiders to build frame and radii of their webs and as lifelines. For both purposes high tensile strength and elasticity are required. The combination of such properties results in a toughness that is higher than that of most other known materials. The dragline silk of Araneus diadematus, for example, demonstrates high tensile strength of 1.9 Gpa approximately equivalent to that of steel (1.3 Gpa) and aramid fibers.
Systems for the recombinant production of spider silk proteins in E. coli have been developed earlier (WO 2006/008163, WO 2006/002827). As an example, it is referred to WO 2006/008163 (claiming priority of U.S. provisional application No. 60/590,196). In this expression system, single building blocks (=modules) can be varied freely and can thus be adapted to the requirements of the specific case. Modules of this type are disclosed also in Hummerich, D., Helsen, C. W., Oschmann, J., Rudolph, R. & Scheibel, T. (2004): “Primary structure elements of dragline silks and their contribution to protein solubility and assembly, Biochemistry 43, 13604-13612”. Further modules are described in WO 2007/025719. It is known from nature that spider silk proteins can be processed into threads. Spiders are experts in using different types of proteins to form silk threads and cobwebs. Technical processes to transform spider silk proteins into threads have been described, for example, in WO 2007/031301.
Synthetic or inorganic materials, e.g. synthetic or inorganic fibers, have been important for the industry for decades. Among these, the aramid thread kevlar, for example, is five times stronger than steel on an equal weight basis, yet, at the same time is lightweight. It also shows low electrical conductivity and has a very high chemical resistance—it is inert. On the one hand, this condition is desired, e.g. for protective clothing. On the other hand, however, aramid threads are very difficult to modify after production. It is nearly impossible to efficiently dye aramid threads or to attach molecules to the surface of aramid threads. Aramid threads are also only slightly wettable. This is also true for other inert synthetic or inorganic materials—they have a high chemical resistance, are only slightly wettable and can not be modified after polymerization under mild conditions, i.e. under low temperatures and without the use of toxic solvents. Common coatings either do not stick well to the surface of inert synthetic or inorganic materials, e.g. inert synthetic or inorganic fibers, and common coatings are not very durable or have to be applied under harsh conditions, i.e. high temperatures and/or toxic solvents.
Thus, there is a need for novel, suitable methods to modify inert synthetic or inorganic materials, e.g. inert synthetic or inorganic threads, after production, preferably at low temperatures and without the use of toxic solvents.
The inventors of the present invention have surprisingly found that the use of silk polypeptides as coating materials provides a highly efficient coating under mildest conditions and enables the manufacture of silk coated inert synthetic or inorganic materials tailored for specific applications. Up to now, very harsh conditions (if at all), like plasma treatment, were necessary to alter inert synthetic or inorganic materials, e.g. inert synthetic or inorganic fibers. There is also no method of coating inert synthetic or inorganic material available employing silk polypeptides, and, more importantly, no one thought about coating existing inert synthetic or inorganic fibers with a layer of pure silk—as this seemed completely impossible so far.
The inventive coating allows attachment of molecules under mild conditions, i.e. under low temperatures and without the use of toxic solvents, to inert synthetic or inorganic materials. In the medical technology, for example, agents, such as pharmaceutical agents, can be efficiently coupled to silk covered inert materials under non-destroying conditions.
In addition, inert synthetic or inorganic materials show several limitations. Most inert synthetic or inorganic materials, e.g. inert synthetic fibers such as aramid fibers or carbon fibers, have an non-desirable surface feel in comparison to natural fibers like, for example, insect silk. This means that most inert synthetic or inorganic materials are harsh, rough and brittle and, therefore, not pleasurable to touch. On the other hand, insect silk fibers share not the characteristics of inert synthetic or inorganic fibers and are often much more expensive.
Therefore, there is a need for novel inert synthetic or inorganic materials having an improved surface feel, i.e. improved haptic.
The inventors of the present invention have surprisingly found that silk coated inert synthetic or inorganic materials, e.g. silk coated aramid or carbon fibers, combine the advantages of inert synthetic or inorganic materials and the advantages of natural materials. Silk coated inert synthetic or inorganic materials have an improved surface feel (haptic) which renders these materials more pleasurable to touch. This effect is accompanied by an optical shine caused by the silk surface, which is more appealing than the dull appearance of aramid fibers or the black surface of carbon fibers. As an example, a kevlar fabric is suboptimal for applications in the near vicinity of or at the skin due to the rough and displeasing surface feel. Silk coated kevlar fabrics, however, are comfortable.
The industry is also constantly aiming to improve naturally occurring materials, such as cotton or wool, to exhibit novel and significantly improved physical, chemical and biological properties and functionalities. Furthermore, for textile industry it is desirable to provide naturally occurring materials, such as cotton or wool, with improved strength, elasticity, bending rigidity and/or resistance to motion while retaining air permeability and wearing comfort.
The inventors of the present invention have surprisingly found that the use of silk polypeptides provides a highly efficient coating reaction which enables the production of coated naturally occurring material having the desired properties mentioned above.
The inventive coating reaction using silk polypeptides also allows the effective attachment of molecules to naturally occurring materials to produce materials tailored for specific applications, e.g. coated textiles, clothing, and textiles for footwear having highly active surfaces providing UV-blocking, antimicrobial and self-cleaning properties.